Evolutionary History
a. The concept of biological evolution
‘Evolution’ in biology is a general term for the understanding we have of how reproduction in organic life works. The core insight, obvious enough to be almost instinctive, is that offspring are related to their parents, and traits of lineages can be selected for, mixed and matched predictably, on purpose by mating choices. Saying it is one thing and comprehending it a different ballpark entirely, moving up to the major leagues so to speak. We of course have engaged in selective breeding for millennia, often at the expense of quality of life and adaptability of the organism, inducing dysfunctions such as a pinched spinal column, itchy skin, stupidity or sterility (in addition to many strains that have clearly been augmented and which might even concur if they could converse and learn our methods). At the same time, we select our own mates based on more intuitional priorities, such as dependability of our partner as a fellow parent, opportunities available to offspring, status in the social group including intrasex status, even pure pleasure, with all the nuances that transcend trait prediction. Bringing the mechanistic and natural approaches into alignment is no easy task.
We would like to know how heredity works well enough to improve upon our choices, sidestepping avoidable errors, warding off disease while enhancing the prospects of progeny. This is the justifiable reason for applications of evolution to human development — knowing ourselves and making ourselves better — but often we presume to know beforehand what we are trying to prove, namely what the superior strategy is and by extension who is superior, usually of our own bent, then set about confirming what we already believe we know anyway by ignoring pertinent facts and neglecting auspicious lines of research in favor of those supporting our agenda. We have to keep in mind that, while hypothesizing is essential to science, the pursuit of knowing is foundational and social conclusions are the speculation, not the more alluring but irrational opposite. As a scientist with integrity, Charles Darwin inaugurated the biological theory of evolution with an impartial treatment of many new facts and uncertainties, not a sales pitch, one of the factors in its enduring relevance, which has been added to quite rationally and systematically by researchers across many generations, though this progress is oft misinterpreted and misused.
One example of erroneousness is the common assumption that since intelligence is a desirable trait, we should employ evolutionary paradigms to facilitate its development through social engineering, in essence assuring that intelligent citizens produce the most offspring and a thereby greater influence by designing institutions which assist these individuals and bring them together so society can benefit from their talents. But this presumes we know what intelligence is, including how it arises via reproductive choice, how intellectual advances are integrated into society (is it really top-down, via a hierarchy of merit?), and how mutations responsible for increased intelligence operate.
Maybe definitions of intelligence differ between cultures, maybe high aptitude under some circumstances tends to be coupled with deficiencies elsewhere, maybe some impositions of intellectual dominance undercut its goals, maybe intelligence can be unpredictable from phenotypes or even the genetic endowments of parents, maybe in vitro and targeted fertilization or genetic modification based on models of intelligence could collaterally corrupt the gene pool, maybe intelligence would be better served by spontaneous diversity rather than premeditated categories, or maybe intelligence chauvinism would make the most intelligent less so and perhaps even irrational. Massive interdisciplinary study is likely necessary to even approach engineering intelligence; the closest we have come is equal access to public school curriculum, but even this most successful movement towards intellectual empowerment has lost steam, at least in the U.S., as crude prejudices about intelligence and much else inhibit cultural relations with conflict.
Clearly, there is more going on than mastering neat and tidy, one-size-fits-all routines of abstraction in any commitment to reaching optimal thought, much more, yet we all believe we know what intelligence is despite ubiquitous ignorance of how cognition functions and transmogrifies in widely varying, rapidly evolving societies. Intelligence is a genetic predisposition, physiological structure, psychological mindframe, array of reasoning procedures, social construct, and we thus far have only superficial models in all these contexts. We typically create definitions of intelligence that please us and enforce them more than peering into its truer and unintuitive nature. What the theory of evolution can tell us about intelligence is going to be part of the scientific marathon of a millennium, but knowing what intelligence is currently seems easy enough, just half-ass it and lie.
We need a commitment to bursting the theoretical conventions underlying our social constructs apart at the seams, deconstructing them in all academic fields, giving birth to a culture of humane optimization that cares what we actually are and can be rather than who has prerogative to own the upper hand, though human nature of course cannot dispense with authority structures nor decorum in communication, but this is simply a matter of collaborating or conferencing, which humanity accomplishes billions of times per day. Knowledge is only such insofar as it achieves a sufficient degree of transparency; severely restricted access is assurance of divergent cultural evolution and the transgressable arbitrariness of our principles, rules and regulations, antithetical to rational convergence and group affirmations of intelligence (but are we already too far down the dead end path?). If we are not careful, intelligence can become a nullity by the very act of inadequately defining it and thus hampering our ability to enrich the definition; setting it in stone, fossilizing it.
A beneficial contribution made by the theory of evolution, as opposed to its coopting for purposes of pseudomeritocracy, is the insight that genetic diversity in a species is actually insurance against the arrival of unfavorable conditions, like diversifying an investment portfolio. Humans often view racial purity as a virtue, drawing a line in the sand between ethnicities and regions, but it turns out that variation is the truer virtue as it pools the genetic resources of all races into culture as experimentation device. Likewise, the preservation of ideas and ideals allows more recombination of ‘memes’- any unit of conceptual expression. Current knowledge of evolution has suggested that traits characteristic of a race or locality are not marks of distinguishment or privilege but rather accidents of the founder effect, an initially small population propagating unique genes as it enlarges due to limited reproductive choice, which are usually tied to disease when concentrated, for most mutation is harmful. Indigenous populations of the Andean mountains in Chile were seeded by such a small group that they all have only type O blood even though their Siberian ancestors had all blood types; this instance is adaptively neutral, merely a minor frustration to blood transfusion, but regional trait deficiencies and malignancies occur across the world. Interracial marriage, beyond being a diplomatic venture, may reduce attrition of trait profiles with diverse recombination and also incidence of unfavorable genes in the world population by dilution more than institutionally guided reproduction or association ever could, in addition to creating novel and rare hybrids of traits with a favorability that has not yet even been categorized, like those contributing to intelligence for instance, or immune system function and overall health.
We see how genetic bottlenecks affect species such as the cheetah, causing their populations to atrophy at an accelerating rate once a tipping point has been reached. Evolution’s concepts account for this in ways that an interpretation of genetics as static never would. Race mixing has been common for much of human history, though usually discouraged during periods of conquest, competition or technological inequities. The theory of evolution can turn us back from colonial and nationalistic prejudices, restoring us to what is probably humanity’s default state: reproductive sharing. This does not diminish the fact that much more research into evolutionary genetics will be productive — perhaps there are some important exceptions — and we should continue analyzing data like competent scientific thinkers, following the evidence wherever it leads and using hypotheses more as provisional means to organize our thoughts than rhetorical claims of value.
Moving away from moralizing and back to science, Darwin’s basic observation, so obvious in retrospect that scientists everywhere were slapping their foreheads, was this: just as human breeding of animals selects traits by facilitating their reproduction and thus transforming populations, so do natural environments, creating a reproductive differential by favoring those most well-adapted to surroundings, a concept he called “descent with modification”. His proposal was that this phenomenon had, over vast amounts of time as indicated by the fossil record, diversified life into all species of the world starting from some kind of common ancestry, fitting form to functional need: “the origin of species”. Mechanisms of hereditary change were largely unknown at this juncture, the 1860’s, but population genetics combined with organic chemistry eventually arrived at models for how mutation and natural selection happen down to the molecular level and up to the macroscopic level of entire ecosystems. Theory is continually enhanced by research to advance our understanding of the links between history, chemistry, physiology, ecology, and hereditary relatedness. The evolution-based interpretation of genetics can tell us volumes about what we are and what we are not as a mutating species with a dynamic and astoundingly ancient nature.
b. Mechanisms of biological evolution
Evolution as we know it proceeds according to principles of natural selection inhering as relatively constant relations between reproducing populations of organisms and their ecosystems. At this time, the dynamics foundational to our evolutionary conceptualizing of phenomena are both the means of exchanging genetic material and the passing of it on to the next generation intact enough to preserve and build upon functionality within the ecosystem/phenotype/genotype complex. Ecosystems exert pressure on an organism to develop a form or type, and genotypes, the total genetic phenomenon, make these types recursive, lasting patterns, lending them their internal coherence. Phenotypes mediate between external environments and core hereditary code as cellular and systemic chemistry within emergent physiology, which evolutionarily proceeded in some lineages from outpacing inclement conditions towards increasing capacity to predict and outmaneuver. Thus, from the perspective of function, organisms are elaborate prediction machines.
While evolution can be conceived linearly in retrospect, as parallel changes over constant flow of time, basically chronologies, the generative spontaneity of ecosystems themselves — environments and interfaces — is nonlinear, supraconceptual even, and as such can diverge in many directions that for the foreseeable future will remain no more precisely modeled than as statistical probabilities. Form does not fundamentally adapt a function, nor does it essentially determine it; perceptions and conceptions of form converge in the human concept of function through our acts of observing and interpreting what we observe. The concept ‘function’ is a value-laden, inference-laden, perception-laden animated entity that participates in creating reality from the depths of its enigmatic existence. The concept ‘evolution’ is linked to its imposition upon patterns within its perspective, a multifarity of predictive and predictable machinations with submerged, perhaps incomprehensible impetus.
All of this merely to say that when analyzing evolution we primarily assess its past, the closest we come to 20/20 vision, piecing together bits and pieces of the puzzle existing within our mental range, but extrapolations into the future are always tentative as usages of conceptualized ‘history’ in the fluxing present. Facts of evolutionary history are theoretical, and postulated mechanisms much moreso; the potential for overhaul must be admitted and degeneration ever guarded against, though some interpretations are without a doubt more justifiable to intellectual integrity — a respect for the entire body of fact — than alternate commitments. We must be bold; we must also be cautious in due measure.
These reservations in mind, we can immerse ourselves in the evolutionary perspective. The first signs of life were probably metabolic cycles, dynamic equilibriums of complex atomic components within watery habitats. Next was an upspringing of membranes in symbiosis with catalysts of chemical pathways and eventually self-replicating molecules such as RNA segments. Replicators, together with metabolic microstructures they depend on for sustainment, were sheltered more and more by membranous buffering and linked to stretches of reaction embedded in molecule-studded membranes themselves. These primitive cells began to divide, a chemically simple, automatic process, and as replicator mutations in separated solutions began to multiply, life diversified. These initial cells formed microscopic ecosystems; they migrated on minute scales with increasingly synced activities, and became specialized for roles in colonies, expediting sustenance of the whole. At first they had preyed, fled, been driven to extinction or attained dominance, but the trend was towards greater integration as organismal conglomerates.
Self-replicators were an ecosystem within an ecosystem, competing for access to metabolic reactions, developing greater ranges of cellular control and merging or diverging when cells combined or divided. As this flurry of microscopic evolution came together, replicator-swapping enhanced the variation of all cells, and while perhaps sometimes toxic, this generally consolidated and accelerated the emergence of genetic stability. The range of genetic variation reached a critical mass and stable enough line of descent that microorganisms began to adopt the form of evolving species. Those with the most efficient adaptations prevailed, becoming the original archaea and bacteria, continuing to gene swap with immediate neighbors through tubular structures specialized for the task, the beginnings of sexual functionality, though at this stage heredity was still haphazardly horizontal, very much unlike the vertical “tree of life” in higher eukaryotes.
Mechanisms of hereditary divergence within populations of prokaryotes and then primitive eukaryotes took shape, utilizing protosexual gene swapping through connective extensions to replicate, altogether winning the race against adaptive demands of environments, at least well enough for the continuity of life’s lineages. Macroscopic reproduction was initially asexual, budding off parts of the organism that grew into mature specimens. Those that could spread farthest came to dominate the evolutionary landscape, and those with the most reproductive activity even more. Initial aqueous ecosystems favored the fecundity of what could be carried on currents to more pockets of nutrient-rich space located at some distance, filiating new populations that underwent genetic drift, a dilating fund of mutation sculpted into dazzling miscellanies of structural and functional variety by natural selection in dissimilar environments.
Seeding new populations asexually in this way was inefficient because the offspring’s chances of success were unreliable, depending on a lucky break that placed them in conducive conditions. Evolution by this method was also slow as offspring were nearly identical to their parent genetically, only differing by small amounts of mutation in what had become very stable, error-resistant genetic systems within their cells. Sexual recombination, however it arose, was a huge leap forward, shuffling the gene pool in each generation by synthesizing DNA contributions from differing sex cells, increasing variation potential within species by many orders of magnitude. This enlarged genetic template was necessary due to the threat of microscopic pathogens that evolve rapidly, infecting and destroying cells in runaway replication, the seizure of cellular machinery and toxin production clearing their way, also as response to predator/prey dynamics. Complex biochemical adaptation gave rise to greater defensive and offensive capabilities: spines, poisons, mouthparts, intercellular recognition mechanisms, and attack of invasive replicators such as viruses and bacteria. Further refinements made organismal structure even more efficient: digestive and circulatory systems for better organized allocation of nutrients, also concentrated excretion of metabolic waste products, endocrine and nervous systems to helm and marshal targeted discharge of overall function, and the like.
Sexual reproduction probably began as a process occurring within single organisms, as is the case for most plants, with sex cells — ‘gametes’ — specializing as sperm for fertilization and eggs for growth. All kinds of mechanisms unite sperm and egg: phallic structures, wind and air currents, vector species acting as germ cell transporters such as bees, all the vast differentiation in nature. It was metabolically expensive to assume every role, fertilizing, growing and distributing offspring, so species by some long and winding route arrived at specialized functions for advanced reproductive success: sexes, genders, maturation and metamorphosis in multistaged life cycles, and eventually as the nervous system progressed, a behavioral and then emotional bond between parents and with their offspring that expedites reproduction in the next generation.
Intraspecies relationships bolster reproductive populations, granting them various forms of communality. Though this socializing can be tumultuous and sometimes self-defeating, as in the case of competitive, aggressive, even infanticidal behaviors, a collection of meanings emerge that lend activity in groups greater predictability for their members, a buffer against any hostility in surrounding conditions if only by virtue of the unassailably more massive scale of their coordination. Social behavior makes hundreds of thousands of species distinguishable by their ritual acts, especially higher animals like birds and mammals, with even reptiles and amphibians having something approaching normalization, though most if not all Earth’s organisms are much less improvisational within their constraints than modern humans. Traits that play the role of social inclusionary medals, conferring rank in a reproducing population, establish fluctuating contours of status regulating delayed gratification of pleasure. In humans, this was sublimated to such an extent that we made a leap to ethics and rationality, the opening of a niche for high functioning self-control, integrating mores and then morality with the evolutionary ascent of our cognition to birth intellectual focus and contemplative restraint as widespread, respected dispositions.
Human language with its relationship to technical and creative conceptualizing is horizontally transmissible from mind to mind at levels approaching gene swapping in prokaryotes, a whole new ‘memetic’ encoding of thought and behavior that is conquering the planet, transitioning the biosphere into an infosphere revolving around technology and its effects. This mechanism of evolution is in its infancy; we are still experimenting with methods of vertical descent via educational traditions for molding the next generation, and ‘memes’ — units of culture — continually flash through populations of human communicators and then out of existence altogether, with only a relative few laying down permanent roots, and even these are transforming radically at all times, though we rarely acknowledge our inconsistencies. Whether humans will master cultural transmission, optimizing life and leisure for ourselves, or be submitted by the vicissitudes of evolving technology and intelligent computation remains to be seen. We will find out if culture will be for humanity’s own long-term prospects within both our individual and collective spheres.
To summarize:
Asexual cell division, molecular self-replication and gene-swapping are the evolutionary mechanisms in prokaryotes and early eukaryotes. Asexual budding from off of macroscopic organisms, reliant for its evolutionary viability on factors such as currents and vector organisms as well as the universality of genetic drift, was the preeminent eukaryotic innovation. Sexual recombination via a vast selection of devices for fertilization, embryonic growth and maturation to reproductive ability made eukaryotes ubiquitous. Division of reproductive labor gave some species a leg up on their competitors, evolving into complex behaviors partially unique to each, a sociality of which human culture is the high water mark. Finally, memetic transmission of human concepts, both horizontally (between individual minds) and vertically (in organized ways to subsequent generations), is still emerging and revolutionizing the nature of our planet as it does, with Homo sapiens’ peaking status as apex species in jeopardy of backsliding, in the worst case scenarios leading to extinction or enslavement by a culture that controls us all, dictating and restricting what we can accomplish.
c. Discovery procedures
In piecing together our evolutionary past, the order of events can be established with high accuracy, but dates themselves are always approximate and up to this point many phenotypic changes largely defy our wish to define them with specificity. Theorists first believed that biological evolution resembles the slow, protracted transitions of geological change, contributing to a paradigm of ‘gradualism’. As we have observed more evolution occurring in both natural environments and the lab, obtained a detailed timeline by ongoing analysis of an expanding fossil record, and altered theories accordingly, it has become evident that ‘punctuated equilibrium’ provides a better model of evolution. Relatively long periods of stability are disrupted by a particularly beneficial mutation or an extinction event, after which adaptive radiation and genetic drift occur surprisingly fast as organisms spread to new environments and develop novel forms in order to fill every possible niche as they exert themselves to survive and reproduce. The way in which this diversification occurs is being assiduously researched, but we have much to learn about how speciation events unfold.
Some reliable scientific techniques exist for unraveling mysteries of evolution. Radiometric dating enables us to roughly determine the age of fossils because the half-life of a radioactive isotope is constant as its small proportion in a bone or tissue sample decays to the common, stable form. Carbon-14 dating is accurate up to sixty thousand years ago due to the constraints imposed by this element’s half-life. The procedure is employed to reliably place evolutionarily recent remains and artifacts of Homo sapiens’ global migrations and seachanging switch from hunter-gatherer society to civilization. Potassium-argon and argon-argon dating have a much vaster range due to longer half-life, accurate from a hundred thousand years ago to the origin of Earth. Prior to development of these dating techniques, geological constants such as sedimentary stratification in rock and soil surrounding a fossil were used. This was extremely restricting as there is much upheaval and erosion in Earth’s crust confusing the record, but there are nevertheless numerous sites around the world where geological strata remained intact enough that even 19th century scientists could construct accurate models. Putting all the past and present data together gives a valid picture of how evolution progressed and some key events in Earth’s biological history.
DNA analysis makes it possible to determine ancestry with extreme precision, sampling tissues of both extant species and preserved specimens of extinct species, then sequencing entire genomes as well as constructing a global family tree. Scientists are continually making new discoveries, tweaking models and interpolating data, but based on two hundred years of collective investigation we know a huge amount about evolution on our planet, and with inclusion of astronomy some basics regarding the universe as a whole.
d. Prebiological evolution
Recent science claims the universe began as an extremely massive and dense concentration of substance that was destabilized, exploding outward in an event called the ‘Big Bang’, producing all the matter in our universe. This explanation is largely based on the presence of background radiation, an all-pervading substrate of the cosmos. As this electromagnetic radiation expands, its wavelengths elongate like a spring, exciting surrounding particles less energetically and dropping temperatures, with temperature simply being a measure of the degree of motion in matter. The current temperature associated with this radiation has been estimated at a couple degrees above absolute zero, and when compared to the effects of radiation that is always being generated on this planet and elsewhere in combination with its very slow rate of stretch, the age of the universe seems to be roughly 14 billion years, with the event horizon of outer galaxies blasting away from their commencement at an accelerating rate.
Nascent matter immediately would have started to aggregate from collisional chaos, and gravitational pull instigated further collisions as well as bringing mass into orbit around much larger mass. It is estimated that after only about one billion years most stars and their solar systems had begun to take shape. Based on likely rates of formation, it is claimed that in the contemporary universe approximately 80% of the stars ever to exist have already materialized.
About five billion years ago our own star reached the stage in its lifecycle where gravitational stability is disrupted and it burst asunder, condensing upon itself internally and blasting its outer matter into space in a cataclysmic supernova. Before this event, the densest matter in our solar system was probably iron plasma in the star’s core, but immensity of unleashed force fused atoms together and created the latter half of the periodic table, including radioactive elements such as uranium. Some of this matter remained in orbit around what had become our sun as a gigantic cloud of fine particles. Debris began to condense around silicate dust, and only .1 billion years later heavy elements, carbon compounds and ice had formed the major planets of our solar system together with the huge asteroid belt surrounding it.
As frictional, explosive heat kept our planet molten, in which state heavier atoms diffused to the core and lighter or gaseous substances centrifuged towards the Earth’s crust, constant barrages of meteor strikes (indicated by a crater-pocked moon) and volcanic activity (covering up signs of ancient meteor strikes on Earth) delivered gas, ice, metal-rich rock, and what would become organic compounds to the surface, providing raw materials necessary for life. During the churning havoc of this ‘Hadean’ era, lasting until 3.9 billion years ago, a celestial object which must have been roughly the size of Mars probably collided with Earth, rending the planet apart and forming our orbiting moon, the gravity of which stabilized wobbling of Earth’s axis as it rotates, and consistent atmospheric conditions, our recurring seasons, became possible. Meteor strikes had mostly ceased prior to 3.8 billion years ago, clear from a lack of craters in large swaths of 3.8 billion year old crust, the oldest rock on this planet. The majority of errant debris in our solar system had plunged into the sun or broken up in Jupiter’s massive gravitational field, atmospheric gases erupting from Earth’s volcanoes accumulated, undisrupted by the violence of meteors, weather patterns took shape, the surface and its oceans were replete with biologically functional compounds and molecules, and the stage was set for chemistry to make a leap to living ecosystems.
e. The origins of life
Figuring out how life came together is not an easy task. To start with, this event is incredibly ancient, leaving no traces of direct evidence about what transpired. We do not know for sure which aspects of life are initial conditions responsible for the possibility of development and the whole host of contemporary forms. We do know at the very least that prokaryotic organisms preceded eukaryotes and evolved into them, supporting the assumption of increasing complexity during pivotal moments, but asexual means of reproduction exist in archaea and bacteria to this day, implying that genetic transmission was first in large measure lateral, a convoluted web rather than a branching tree of lineage like we see in later eukaryotic domains.
The exact conditions are inaccessible to us, but we can perhaps arrive at a simulacrum of the beginning by tracing prokaryotic lineages backwards as much as possible, assessing currently hospitable environments for single-celled organisms and which of these ecosystems might have been present on inorganic Earth, then examine the way in which molecules self-organize into organic structures. We can run experiments based on the most likely hypotheses and see if some variety of self-sustaining life does come to fruition and how it progresses. This can tell us a lot about organic ecosystems even if it turns out we can never be positive how closely our replica conforms to the actual past or what constitutes the total range of possibility.
The first models of early Earth’s environment speculated that it consisted of ammonia (NH3), methane (CH4), water (H2O) and hydrogen gas (H2). One prospecting scientist gained the opportunity to run a laboratory test that would determine whether inputting energy into these ingredients generates something akin to organic form. The four substances were sealed in a vessel and sparks of electricity applied, simulating the scenario of lightning strikes. It was found that organic molecules such as amino acids and nucleotide components can be synthesized. This was a long way from living organisms, but spontaneous transformation from the biochemically simple to the more complex was manifestly plausible.
Some more experiments were undertaken that uncovered suggestive phenomena. It was discovered that when phospholipids with a single hydrophobic tail are synthesized in aqueous solution, they self-assemble into phospholipid bilayers shaped like spherical bubbles, about the dimensions of a typical cell membrane. They are semipermeable, though much more porous than a living membrane, and separate the solution into internal and external concentrations, even pinching in two on their own when occupied. It did not prove as possible to coax molecules into the form of a two-tailed phospholipid from which modern cell membranes are composed, but when these larger versions were introduced whole into solutions as well they automatically integrated into coalescing membranes as less permeable and more structurally stable subunits six times as long. However double-tailed phospholipids arose, it was obvious that it would have been an instant advance, making membranes capable of greater self-regulation by supporting much more selective mechanisms of permeability via embedded molecules such as proteins, sugars, all kinds of functional macromolecules.
Various organic substances were added to solutions to see what they would do. Sugars were included with genetic material, enzymes and protein, from which it was found that a range of macromolecules can be synthesized independent of the structurally complete cell; it took some theorizing effort to achieve successful combinations. Exposing solutions to UV radiation seemed to streamline enzymatic processes, increasing yields of sugar while minimizing aberrant byproducts, the ‘random noise’ in the system. Salutary effects of UV light at this basic level were a significant discovery considering how essential photosynthetic processes are to current ecosystems, intimating a possible origin of life in shallow pools near land surfaces.
Despite centrality of photosynthesis to the biosphere, it was found that prokaryotic organisms survive in many climes, wherever energy can be harnessed by chemistry. Competing theories were formulated that considered the possibility of extremophile prokaryotes as the first lifeforms, a likely option considering turbulent conditions of an Earth that, 3.8 billion years ago, was still in volcanic apoplexy. It became common currency that early Earth was profuse in nitrogen and carbon dioxide gas as well as water, with paltry amounts of ammonia and methane, conditions prevailing in the interface between the atmosphere and Earth’s surface, but due to chaos induced by tectonic shifts and magma emissions, it seemed likely that life would have burgeoned in more stable deep ocean environments, though an energy source had to be available.
This led scientists to deep sea hydrothermal vents surrounded by teeming populations of single-celled life. Near boiling ocean water heated by the molten mantle beneath Earth’s crust froths around fissures that inject hot gas as well as simple subunits of macromolecules such as amino acids and other carbon compounds into nearby rock, eroding microscopic pores within their bulk. Chemicals circulate in and around these tiny chambers that act like nodes between wormlike tunnels connecting this collective chemistry to the outer ocean. It is postulated that dissolved gases such as carbon dioxide, hydrogen and nitrogen, as well as organic molecules and proton gradients from hydrogen atoms stripped of electrons all subsist in the supercharged environment, conditions that may be sufficient to produce a metabolic cycle without the presence of membranes, as networks of linked pores could be the total requisite structure, functioning like a congregate of cell walls. Metal ore surfaces exposed within the chambers may catalyze energy transfer, acting the role of primitive enzyme. It is an intriguing model, one that seems to explain what could be bacterial descendants living in droves nearby, and scientists recently committed to testing it.
An experiment was designed that placed a solid clay brick with microscopic pores and channels in a sealed cylinder of aqueous solution. A tube pumped heated flow of water through the clay in such a way that circulation was achieved, and further tubing was assembled to introduce gases and organic molecules to the solution in a concoction that mimicked postulated conditions of hydrothermal vents with high fidelity. Scientists planned to set the apparatus in motion and determine whether larger molecules can be formed. Efforts such as this may disclose much about how life may have irrupted into existence.
It is not hard to imagine a sort of membranous biofilm adhering to interiors of the rock, becoming studded with as well as inhabited by macromolecular clusters conjuncted to the nutrient rich cycle, then differentiating into primitive cells that expanded in range, complexity and diversity as the first prokaryotic lifeforms. Each evolutionary step is improbable on its own, but metabolic self-sufficiency together with mutational self-replication only had to materialize once or rarely, and billions of years of naturalistic trial and error in prokaryotic time is like a macrocosm of the universe. It is exciting to envisage what we might discover and create.
f. Organic evolution
The first single-celled organisms appeared no later than 3.8 billion years ago, providing evidence of their existence as fossils in the world’s oldest sediment. They organized into colonies called stromatolites that still exist today, puffy tissuelike surfaces in and around rocks, where it is thought division of labor and specialization originated. Some of these primordial cells captured energy, some offered protection, and still others adhered the entire mass to stone, among more functions. As already mentioned, these cells also engulfed each other at times without annihilation, forming symbiotic relationships from which the eukaryotic domain originated.
Organisms remained simple for billions of years as the Earth’s climate was harsh, alternating between extremely hot and cold epochs. Temperatures at Earth’s surface were largely determined by the amount of carbon dioxide in the atmosphere along with minute concentrations of additional greenhouse gases, mostly erupting from volcanoes and accumulating for hundreds of millions of years. As volcanic activity abated due to a stabilizing mantle of molten magma, probably settling down from massive collisions with celestial objects, carbon dioxide became fixed in limestone deposits on the surface much faster than it entered the air; these layers of limestone still exist as geological strata in some parts of the world. Depletion of carbon dioxide combined with a sun burning less intensely than it does today led to severe cooling, which happened three times in periods called “snowball Earth”. The entire surface area of the planet was encased in ice for hundreds of millions of years, revealed by glacial rubble we know would have been at the equator when we extrapolate tectonic plate shifting back in time to these ancient dates. The only part of the planet not ice-coated was in and around active volcanoes.
Carbon dioxide was still being spewed into the atmosphere and gathering in high amounts but had no greenhouse effect because, even though limestone production had halted, radiation mostly reflected off the ice and back into space without generating much heat. There may have been a small amount of melting from direct contact with the electromagnetic spectrum, but not enough to counteract generally frigid temperatures. The exception was land surrounding volcanoes: heat caused glaciation to gradually recede, and exposed land absorbed sunlight, emitting infrared radiation in response, which in turn heated up greenhouse gases in the atmosphere. When enough land had been unveiled, the cycle reached a tipping point and melting accrued relatively fast, transitioning the planet to a considerably hot environment for hundreds of millions of years, during which limestone began to form again, leading to the next snowball Earth phase. The last age of frozen Earth lasted from 1.5 billion to 600 million years ago according to geological data.
During these fluctuations, life was probably sheltered in Earth’s oceans, the temperatures of which did not range to such extremes, providing a buffer against atmospheric change. A worldwide melt would have generated much more water; whether life made the next developmental leap in a newly formed lake, an ocean or a shallow is not known, but about 550 million years ago a huge upgrowth in eukaryotic lifeforms occurred as recorded in fossilization, probably fueled by expulsion of oxygen gas waste from photosynthetic organisms as carbon dioxide was harnessed by chlorophyll pigments, coupled to emission of carbon dioxide waste by oxygen-utilizing respiration of diversifying prokaryotes, plants and animals. This cycle of gas consumption attenuated atmospheric buildup and depletion, in addition to facilitating formation of the ozone layer (O2 + high frequency radiation = O3). Combined with more intense UV light from a sun that had become hotter, which compensated for a depressed maximum of greenhouse gas concentrations due to globalized metabolizing of carbon dioxide, the environment’s temperatures and radiation exposure stabilized at levels accommodating diversification of lifeforms within niches suddenly available everywhere. In response to life’s new opportunities and interactions within differentiating ecosystems, selection pressures of the planet were transformed by an overall race to the top of the food chain, the ‘Cambrian explosion’.
The first macroscopic animals were small worms of no more than a couple inches in length and the ediacarans, fleshy amorphous blobs that subsisted in oceans everywhere. The structure of these early ecosystems and their metabolic cycles are a mystery, but we know a huge conversion took place when creatures with exoskeletons developed jointed limbs for increased mobility and primitive jaws for predation and consumption. For these arthropods, watery environments were a feast of sedentary, defenseless ediacarans and primitive photosynthetic matter, feeding what would be the ancestors of crustaceans, insects and arachnids. Sometime between 550 and 250 million years ago, a small worm species underwent a mutational event that quadrupled its DNA, forming the genetic chassis upon which the vertebrate phylum would be founded.
Original swimming organisms were descendants of this ancestral worm, initially having a lampreylike form, then evolving jaws, a backbone for stability and motility, and more acute sense organs, a lineage which gave rise to the first primitive fish. All of these lifeforms — photosynthetic prokaryotes and eukaryotes, predatory single-celled protozoa, arthropods, worms and early vertebrates — began making a transition to dry land. Plants, with the maximum benefits offered by sunlight, were probably the first macroscopic organisms to make a move from shallow marshes to increasingly arid habitats. Arthropods and worms were next, becoming flying and burrowing organisms respectively, utilizing the unfilled niches of air and soil. Then came a splitting off from the fish lineage of amphibians, increasingly adapted to insectivorous behavior above water. By 300 million years ago, all kinds of insects, some with one and a half foot wingspans, could be found soaring above land surfaces everywhere, while arthropods scurried along the ground foraging, linked in the food chain to increasingly water independent creatures of amphibian ancestry.
The timeframe from 350–300 million years ago is called the Carboniferous period. Due to less respiration, oxygen content in the atmosphere was very high, about 35% compared to 21% today. It is a volatile chemical, leaving geological record of its reactions with metals in the soil, and large concentrations were toxic to some organisms, with there being arthropod fossils that show evidence of oxygen burns. However, it was a huge benefit to the evolution of flight since O2 is a relatively massive molecule compared with other atmospheric components such as N2, H2 and CH4, enabling delicate wings to support large insects that in low lying areas of slightly less, nontoxic oxygen concentration functioned as effective predators, while supplying greater amounts of energy to primitive physiological systems of gas exchange. 250 million years ago, O2 concentration reached an extremely reduced level, 15%, which must have paralleled sizable reduction in photosynthesis from some unknown cause; 95% of species failed to survive the ‘Permian extinction’. But by this time four-legged animals called dicynodonts bearing resemblance to modern reptiles and mammals in body design had evolved, and though a bottleneck in speciation occurred, the lineage of mammals soon began and lystrosaurus roamed the land, a pig-sized, reptilelike omnivore that would sire the dinosaurs.
200 million years ago, crocodiles and the first small dinosaurs called archosaurs evolved. From this time until 65 million years ago, dinosaurs and mammals lived together and diversified, mammals remaining small and dinosaurs evolving to the leviathan proportions so fascinating to us today. Lizards and snakes also began to develop and evolve from their common ancestor. 65 million years ago an asteroid strike probably caused the famous extinction of the dinosaurs, clouding the atmosphere with debris and cooling temperatures for thousands of years. Only small organisms with low metabolic needs could survive, including mammals and the smallest dinosaurs which developed coats of fur and feathers respectively to cope with the cold, alongside a remnant of coldblooded creatures such as reptiles, miniature insects and assorted other small species. Mammalia diversified and grew in size to replace dinosaurs as the apex phylum, while small, feathered dinosaurs gained increasing flight capability and transitioned into birds. As the Earth warmed, reptiles also made a comeback, spreading to all corners of the world. Insects, worms, along with innumerable near-microscopic and microscopic organisms carried on, and of course photosynthetic plants and bacteria remained the foundation of all ecosystems. Around 50 million years ago, primitive primates split into the persimion lineage which evolved into organisms such as bush babies, and the early ancestors of anthropoids that would become monkeys, great apes and the Homo genus, setting the stage for our own evolution.
g. The evolution of Homo sapiens
The first anthropoids resembling anatomically modern humans were the product of a switch to bipedalism, walking on only the hind limbs. This change probably took place in at least a few increments as our ancestors moved from occasionally standing in trees to residing full time on the ground, compelled by an environment in which tree habitats were becoming scarcer or food availability was greater at the terranean surface. The eventual move to grassland and plains dwelling may have been dangerous at first as this is where most large predators reside, which would have exerted selection pressure for increased physical size and social cooperation. Alternatively, the physiques and minds of modern anthropoids such as humans, bonobos, chimpanzees, gorillas, orangutans, baboons and gibbons may have arisen from social selection pressures within and between groups, with little sculptive influence from predatory threats.
Our bipedal precursors did not initially have a cranium to body mass ratio much variant from modern day chimpanzees, and archaeological finds indicating tool construction are absent from sites of their fossilized remains. These walking chimplike animals such as Australopithecus made a big leap forward in intelligence when cranium size to body mass ratio and subsequently brain power increased, templating more cognitive functionality and resulting in signs of primitive tool construction, rocks that must have been methodically cracked into sharper pieces.
Homo habilis was the first member of our genus to leave clear signs of technology, sharpening rocks in rudimentarily technical ways about 2 million years ago (what would become the Homo genus diverged from the chimpanzee and bonobo Pan genus 5–12 million years ago). Homo ergaster evolved next and began migrating throughout the world. As this species moved it encountered new environments demanding novel technological solutions, underwent social development, and experienced genetic drift from which the natural, psychological and sociological circumstances selected new traits, generally encouraging creativity, problem-solving and self-awareness. By about 1.8 million years ago Homo erectus, a cognitively advanced species, had spread throughout Asia, leaving spearheads and chiseling tools in its range, and a still more advanced species called Homo neandertalensis, the Neanderthals, had occupied much of the Middle East and Europe by about 1 million years ago, displaying behaviors such as burial and showing the marks of sophisticated handheld tools as well as group hunting of big game. Altogether, nine Homo species have been identified by researchers.
Multiple representatives of the Homo genus were in dynamic equilibrium until at least two hundred thousand years ago, when Homo sapiens evolved; paleontology suggests this may have taken place in either East Africa or Asia. There was some interbreeding between Neanderthals and early humans, but the trend was for Homo sapiens to displace or absorb other Homo species. In what proportion this was due to social and environmental pressures or warring — cooperation, indirect or direct competition — is not clear, but cultures of Homo sapiens were undoubtedly more refined, with the first visual art such as cave painting, jewelry, finely crafted tools and pottery, as well as additional characteristics of hunter-gatherer cultures.
Migration of Homo species took place during intermittent ice ages when sea level could drop as much as one hundred feet and vastly more land was available. If the “out of Africa” theory is correct, intercontinental wanderlust reached modest levels at least 120,000 years ago, but migrations that put down substantial roots began about 60,000 years ago as humans travelled up the coastline of the Nile region into the Middle East and across the Red Sea, relying on shellfish harvests for food and breeding prolifically as they went, at levels that actually increase while on the move (most hunter-gatherer tribes relocate frequently in all situations, responding to changes in food accessibility and seasons). When the most recent ice age began 50,000 years ago, peaking at 20,000 years ago and ending about 10,000 B.C.E., humans journeyed extensively, filling Europe, the Middle East and Asia, even crossing a land bridge from Siberia into what is modern day Alaska and populating the Western Hemisphere as well, replacing dwindling Homo species in the Old World and conquering most of the globe. A maritime culture with advanced canoeing existed in the Indonesian islands, and almost the entire South Pacific was inhabited by about 20,000 years ago. Expanding oceans around 15,000 to 10,000 B.C.E. somewhat isolated these groups, but by 1,000 C.E., New Zealand regions, the last lands untouched by human hunter-gatherers, had been reached.
The epicenter of crop-raising, animal domestication and settlement was the Middle East’s fertile crescent, located between the Tigris and Euphrates rivers in what is modern day Iraq. This lifestyle had begun to spread widely by about 10,000 B.C.E. as temperatures warmed and fertility of land increased. Thawing northern and southern regions adopted farming as well, and culture gradually became more sedentary, organized around enlarging villages and then towns and cities as surplus food grew populations. Social hierarchies became entrenched, war for in-demand resources galvanized sprawling populations against each other within their traditions of authority, and an imperialistic way of life based on cultural rivalries and alliances existing to this day implanted itself as the standard for civilized organization, solidifying into near universality from about 6,000 B.C.E. to the common era.
